Brush the cake with 3 tablespoons apricot jam. Set aside 175 g (6 oz) marzipan and use the rest to cover the cake, smoothing it over the top and down the sides. Trim to fit. Place the cake on a cake board.
Knead the icing lightly and roll out on a work surface dusted with icing sugar until large enough to cover the cake. Dampen the marzipan, then lift the icing carefully over the cake, supporting it with a rolling pin. Smooth over the top, then down the sides. Trim off the excess icing. Colour icing trimmings with red food colouring to make holly berries. Leave to dry on baking paper.
Wrap a rolling pin with nonstick baking paper. Melt the chocolate in a heatproof bowl over a pan of gently simmering water. Drizzle the melted chocolate quickly back and forth across the rolling pin. Allow to set.
Divide the reserved marzipan into 3, colour each third a different shade of green and place in plastic bags to prevent drying out. Lightly knead two green shades together to give a marbled effect. Roll out thinly on a surface lightly dusted with icing sugar and cut out ivy leaves. Mark with veins and twist slightly.
Roll out the remaining coloured marzipan and cut out light green and dark green holly leaves. Mark with veins and shape. Leave all the marzipan shapes to dry overnight on baking paper.
Use a dampened paintbrush to moisten the icing so you can attach the leaves and berries in a wreath shape. Peel the chocolate off the paper, break into 'twigs' and place on the wreath.